A multi-user multiple-input and multiple-output (MIMO) communication system in which a base station uses multiple antennas at a time to communicate with multiple terminals is known. As for the multi-user MIMO system, a method is known in which the transmitting base station determines transmission weights corresponding respectively to the multiple terminals and weights data signals according to the transmission weights so as not to cause any interference between the users (terminals). The method is referred to as the zero forcing (ZF) method. In the ZF method, the transmission weights are determined by using the channel matrices each between the base station and each of the multiple base stations. In other words, a transmission weight with respect to a terminal is determined such that the transmission weight is orthogonal to a current channel matrix with respect to a terminal that is a subject of interference.
When a terminal move or obstacles around the terminal move, the propagation environment between the base station and the terminals varies and accordingly the channel matrix varies every second. The varying channel matrix (hereinafter, the “varying channel”) may disrupt orthogonality between the transmission weight and the channel matrix and accordingly cause interference between the users.
There is a technology referred to as the null-space extension method as a technology for suppressing the effect of varying channels. In the null-space extension method, a transmission weight with respect to a terminal is determined such that the transmission weight is orthogonal to not only a current channel matrix but also a past channel matrix with respect to a terminal that is a subject of interference. This hinders disruption of the orthogonality between the transmission weight and the channel matrix, which makes it possible to suppress interference between users resulting from the varying channels.    Non-Patent Document 1: Iwakuni, Maruta, Ohta, Shirato, Arai and Iizuka, “Jihendou Kankyouka ni okeru Maruchi yuuza Massive MIMO Nurukuukankakuchouhou (Massive MIMO Null-space Extension Method under Time-varying Environment), Shingaku Gihou, RCS2015-17.    Non-Patent Document 2: T. Iwakuni, K. Maruta, A. Ohta, Y. Shirato, T. Arai and M. Iizuka, “Inter-User Interference Suppression in Time Varying Channel with Null-Space Expansion for Multiuser Massive MIMO,” in PIMRC2015, 2015.
In the conventional null-space extension method, the number of samples of current channel matrix and past channel matrix to which the transmission weight is orthogonal is fixed at a given value and the throughput may decrease depending on the fixed number of samples.
This aspect will be described with reference to FIG. 1. FIG. 1 is a diagram for explaining a problem of the conventional null-space extension method. In the example illustrated in FIG. 1, a base station has eight transmission antennas. In the example illustrated in FIG. 1, the base station transmits a data signal to a user #1.
As illustrated in FIG. 1, in the ZF method, the maximum number of users for multiplexing is eight. On the other hand, in the null-space extension method, when the number of samples of current channel matrix and past channel matrix with which a transmission weight is orthogonal is 2 (fixed value), the number of users for multiplexing is four (fixed value). Furthermore, in the-null space extension method, when the number of samples of current channel matrix and past channel matrix with which a transmission weight is orthogonal is 4 (fixed value), the number of users for multiplexing decreases to 2 (fixed value).
As described above, in the null-space extension method, the larger the number of samples of current channel matrix and past channel matrix with which a transmission weight is orthogonal is, the more the number of users for multiplexing in MIMO communications decreases and accordingly the throughput decreases. On the other hand, the smaller the number of samples of current channel matrix and past channel matrix with which a transmission weight is orthogonal is, the more the number of users for multiplexing in MIMO communications increases; however, this easily disrupts orthogonality between transmission weight and channel matrix and accordingly cause interference between the users, which may cause interference between users.